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DNA from the fingernails of Dunedin woman Karin Ann Ross was 430 times more likely to have come from the man accused of killing her than anyone else in the country, a High Court jury has been told.

Former Spotless Cleaning Services employee Alexander James William Merritt is accused of the murder of his 51-year-old boss, whose lifeless body was found in the company car park early on December 2 last year.

The 21-year-old denies the charge.

When Merritt was interviewed by police days later, they noticed scratch marks on his face, which he said had occurred while he was sleeping.

Under pizza boxes, drink bottles and other waste, officers found a dark grey hooded top. Inside its pocket was a hammer wrapped in a cloth, along with a blood-soaked glove in another pocket.

Ms Opperman said the primary DNA from a sample taken from the hoodie was 1 million million times more likely to belong to Ms Ross than anyone else in New Zealand.

A secondary DNA profile was 500,000 million times more likely to have come from Merritt, she told the jury.

Anything more than a million was considered "extremely strong" scientific evidence.

Under cross-examination, Ms Opperman confirmed she was unable to say how DNA arrived on a surface or when it was deposited.

"Given the top was in the wheelie bin with waste, food and whatever else . . . there's every possibility Mr Merritt's DNA could have come from transfer other than directly from him," defence lawyer Anne Stevens said.

"It could have been saliva from something he ate," she said.

Ms Opperman accepted if it was Merritt's biological cells on the garment, they could have been transferred from another item.

A pair of the accused's shoes were also seized by police for forensic examination.

Though there were only minuscule amounts of blood present on one, Ms Opperman said it was 20 million times more likely to have come from Ms Ross.

"You can't say to this jury with 100 percent certainty that that's Ms Ross's blood on that shoe, can you?" Mrs Stevens asked.

The scientist accepted her testing was not conclusive, but stressed the statistical likelihood of her findings.